Meeting Abstract
Placement of Xenacoelomorpha in the tree of life remains a major unresolved question the study of deep animal relationships. Two conflicting hypotheses are under debate—Xenacoelomorpha either form a clade sister to all other animals with bilateral symmetry (=Nephrozoa, i.e. protostomes and deuterostomes) or a clade inside Deuterostomia. Xenacoelomorpha, comprised of Acoela, Nemertodermatida, and Xenoturbella, are bilaterally symmetrical marine worms that lack a number of features common to most other bilaterians, e.g. anus, intestinal lumen, excretory organs, and circulatory system. Thus, depending on the phylogenetic position of this clade, these animals are pivotal for understanding the initial acquisition of nephrozoan features, or they have undergone substantial secondary loss. Here, we have sequenced eleven novel xenacoelomorph transcriptomes, and show robust phylogenomic support for Xenacoelomorpha as the earliest branch of Bilateria. Systematic testing under different models of evolution, as well as 24 experimental datasets designed to address potential artifacts, such as long branch attraction, saturation, and missing data, all corroborate this result. Our phylogenomic analyses support previous hypotheses that the last common ancestor of bilaterians was a benthic acoelomate worm with a single gut opening and without nerve cords. Thus, common bilaterian features such as excretory organs, coelomic cavities, nerve cords and body segmentation were not present at the cnidarian-bilaterian split.
